Trollope said that, after money in the bank, a grudge is the next best thing. His is an observation that is as true in international affairs as it is in personal life, and the United States is a striking example of it. America rarely overlooks an insult, and never closes the door entirely on a past defeat or humiliation unless the perpetrator has in the meantime been crushed. Thus the "axis of evil" made little sense in its grouping of three very different societies, and even less in its implication that they were somehow allies. But it made eminent sense as a grudge list.

The phrase was used in a speech focused on dangers ahead, but in truth it was as much about the past as it was about pre-emption and the future. All three countries had imposed notable defeats on the United States. North Korea, with the help of China, sent American forces reeling back from the Yalu half a century ago. Iran threw out the Shah, who had retained power in that country with the assistance of Britain and the United States, and brought in a regime that added to America's humility by taking its diplomats hostage. Iraq defied the United States over Kuwait, and Saddam Hussein, against what appeared to be the odds, then recovered control of most of his country, resisted American pressure to disarm and made the United States look ineffective and foolish.

These things rankled with many powerful Americans. Others, by contrast, took them as evidence of the limits of the possible, as the result of mistaken American policies, or even as indications that these "enemy" countries might be to some extent in the right.

This division in the American political mind has never been more important than at present. It is the critical issue in the presidential campaign; and the most significant difference between President Bush and his likely opponent, John Kerry, is that they are on different sides in this argument.

The American instinct for revenge, evident also in the treatment of Vietnam, Cuba, Nicaragua and, for many years, China, is more marked than that of other powers. Perhaps America's lack of the experience of defeat on its own continent made reverses in the wider world especially difficult to swallow when they inevitably came. Other nations have reconciled or compounded with those who had defeated them, as the British did with the Boers and the Irish, or the French with the Algerians. The United States, too, has been capable of magnanimity, as with Germany and Japan, but here a generosity of spirit arose only in the context of a total defeat of those two countries and the rapid transference of hostile feelings to the communist states.

America's historic reluctance to be satisfied with anything less than complete victory is now being tested in Iraq, where it seems inevitable that the full conservative programme will not be pushed through, although how far it will fall short of the ambitions of men like Richard Perle and Paul Wolfowitz has yet to be seen. If, for instance, America does not get the right to base substantial forces in that country indefinitely, and Iraq wobbles off on a more or less independent path, the scene may well be set for another of those "Who lost the war?" dramas that have punctuated American political life since the Chinese communists ousted their nationalist foes in the late 40s. That will be especially so if by that time President Kerry rather than President Bush is in charge.

These arguments have always circled around two propositions. On the right, which at times has included the Democratic right, the proposition has been that if only the United States had exerted its full strength, it would have prevailed. On the left, which has sometimes included Republican realists, the proposition has been that there are objectives that are not morally defensible and others that may be desirable but are not practically possible, and that it behoves a great power to recognise when either of these situations arises.

The real importance of the Vietnam war in the presidential campaign does not lie in the contrast between the service records of Bush and Kerry. That contrast may not, anyway, be as great as some would like to make it, given that Kerry's exposure in action was brief, and that Bush, like all trainee pilots, was taking some risks in flying combustible, inherently dangerous military jets. But it is not what the two men put into the Vietnam war but what they and their supporters took from it that matters most.

For old-fashioned conservatives such as Cheney and Rumsfeld, Democratic hawks such as the late Senator Henry Jackson or neoconservatives such as Wolfowitz, Vietnam was a failure of American will. It was of a piece with that earlier failure in Korea, with accommodation with China (even though that was the work of Nixon) and with the error of detente with the Soviet Union.

Under Ronald Reagan they tried to put steel back into American policy, and flatter themselves to this day that the USSR would not have collapsed as it did if they had not done so. Clinton, as they see it, steered the US back toward the path of temporisation and appeasement, including appeasement of forces hostile to Israel. But Kerry was one of the young veterans who saw how the Vietnam war had its origins in a kind of absolutist anti-communism, and that it was not only the war but the attitude behind it that should be repudiated. The question is with America again in a new form today.

Nearly 30 years after the Vietnam war ended, an American veteran organisation has just begun work to survey all the unexploded bombs, mines and other ordnance that still lie around the countryside and cause the deaths of hundreds of Vietnamese a year. An honourable recompense, and some Americans clearly want and need reconciliation with the peoples who have suffered in wars for which the US bears much, although not all, responsibility.

But the Bush administration, while not particularly hostile to Vietnam, lives in a different history. It may be prepared to soften its line on disobedient allies such as France and Germany. But its reluctance to give Libya the benefit of the doubt, its closed views on Cuba, its distaste for unavoidable co-operation with Iran, its view of China as a future rival and its crablike approach to negotiations with North Korea, are all indications of how long-lived the American grudge can be.

Apart from the principle that American power should when necessary be exerted to maximum effect on enemies that the US clearly discerns as such, there is something else at work here. That is the idea that if a regime has stood in the way of the US and got away with it, it should sooner or later have to pay for its temerity.